The mechanism of chromosome movement during nuclear division is a major problem of biology that remains unsolved. The proposed research should provide findings that will clarify the structure and function of the fibrous components of the intranuclear mitotic spindle, will apply to mitotic mechanisms in general, and perhaps to microtubule involvements in other types of cellular motility. The dynamics of micronuclear spindle function in living cells will be determined by quantifying movements of chromosomes and the birefringence of spindle regions during the course of mitosis. The high sensitivity rectified polarizing microscope will be used to make birefringence measurements during the normal mitotic process and during exposure of cells to changes in temperature, D2O, and other mitotic inhibitors. The microtubule distribution profiles will be determined for spindles at different mitotic stages by electron microscopy. Microtubule lengths will also be determined from serial longitudinal sections of the parallel microtubule arrays which characterize the intranuclear spindles to be studied and by the study of thick sections and isolated spindles with high-voltage electron microscopy. The presence of actin filaments within the intranuclear spindle will be determined by S-1 decoration of glycerinated cells, embedded and thin-sectioned for electron microscopy, and by observation of S-1 treated, isolated, intranuclear spindles following negative staining. Force generation in mitosis by microtubule polymerization and microtubule sliding will be investigated by experimentation with spindle models produced by gentle detergent-lysis.